1. Field of the Invention
The present invention relates to an electrochemical cell having a mass flow field made of vitreous, or glassy carbon. This electrochemical cell is particularly useful in converting anhydrous hydrogen fluoride to dry fluorine gas, although it may also be used in converting other anhydrous halogen halides, in particular, hydrogen chloride, hydrogen bromide and hydrogen iodide, to a dry halogen gas, such as chlorine, bromine, or iodine.
2. Description of the Related Art
It is not possible to make fluorine from aqueous solutions due to thermodynamic considerations. The production of oxygen from water occurs at lower potentials than the production of fluorine. Thus, water reacts to form oxygen prior to the formation of fluorine gas. To produce fluorine, electrolysis in a non-aqueous solution is performed. Pure liquid hydrogen fluoride (HF), as anhydrous HF, does not have any water, and hence, the reaction to produce oxygen is avoided. However, anhydrous HF has very little ability to conduct ions, as evidenced by its very low electrical conductivity. Hence, to increase the conductivity of a charge-carrying solution, potassium fluoride (KF) is added to the melt. Therefore, existing electrochemical cells to make fluorine have a melt (non-aqueous) solution of HF and KF. The KF is present to add conductivity to the solution and prevents the use of excess voltage in an electrochemical cell.
Existing electrochemical cells for fluorine evolution use a carbon anode, from which the fluorine evolves. Hydrogen is evolved from a cathode made of either an alloy of nickel and copper, sold under the trademark MONEL.RTM. by International Nickel Co., Inc., or stainless steel. To avoid detonations from the combination of fluorine and hydrogen in the cell, they are separated from each other by a metal shroud which dips down into the melt and hence provides separation the hydrogen and fluorine as they are produced. The cells are initially charged with a HF-KF mixture, then HF is added to the melt to provide for the hydrogen and fluorine that are removed during production. The current density of the reaction in such cells generally ranges between 70-200 mA/cm.sup.2. Such cells are generally operated at 60.degree.-110.degree. C.
The fluorine produced by such cells is often used in converting uranium tetrafluoride to uranium hexafluoride. Uranium hexafluoride is used in the gas diffusion process for making nuclear material. However, the price for fluorine can be quite high. Moreover, both hydrogen fluoride and fluorine are extremely corrosive to both the commonly used materials of construction in fluoride production processes, as well as to human skin. Any leakage, even that by slow permeation of such materials of construction, poses severe maintenance cost and personnel safety concerns.
Vitreous, or glassy carbon, has been used for a current collector in a secondary battery. For example, in U.S. Pat. No. 4,048,394 to Ludwig, a secondary battery which has a current collector comprising a graphite body having a coating of vitreous carbon. The anode and the cathode are molten reactants. U.S. Pat. No. 4,497,882 to Mikkor discloses a sheet of graphite foil that is coated with an amorphous pyrolytic or glassy carbon to fill any openings in and/or through the graphite foil. A thin layer of aluminum metal is coated onto the graphite foil. The aluminum metal coated side of the graphite foil is bonded to an aluminum surface of an electronically conductive material. However, neither of these patents discloses a mass flow field formed of vitreous or glassy carbon and, in particular, in a cell for making fluorine.
Thus, there exists a need for a less costly fluorine production process which utilizes materials which are able to withstand an attack from corrosive hydrogen fluoride and fluorine.